"People talk about the Big One. This is it," said Tom O'Rourke, a Cornell professor of civil and environmental engineering and a member of the federal Advisory Committee for Earthquake Hazard Reduction. "This is what the Big One looks like. We've had an imaginative idea of what the Big One would be like if it struck a major, populated, modern society."

Japan's nightmare comes in the wake of two other events that scientists found surprising in their location and intensity: the highly destructive earthquake in Christchurch, New Zealand, on Feb. 22, which was triggered by a little-regarded fault; and the tsunami-spawning Sumatra earthquake Dec. 26, 2004, on a trench not considered likely to cause such a "mega-quake."

It may seem as if there are more natural disasters these days, but the real issue is that there are more people and more property vulnerable to the violent forces of Earth. Natural disasters are supplemented by technological disasters - last year's Deepwater Horizon oil spill in the Gulf of Mexico being one example. Disaster planners in the United States have to ask themselves how they would deal not only with the obvious types of calamities - Gulf Coast hurricanes, for example - but also the events that are of low probability but come with high consequences.

"You don't get to pick the next disaster. You don't necessarily know where the threats are," W. Craig Fugate, director of the Federal Emergency Management Agency, said this week as he contemplated Japan's horrific combination of catastrophes. "We plan for the things we know, but we also plan for the things we don't know."

People debate what qualifies as a black swan. Most alleged black swans turn out to have obvious precursors and warning signs - the Sept. 11 attacks included. Nothing comes out of the blue, truly.

The next big disaster could be something off the radar of most Americans. A solar flare, for example, could trigger a geomagnetic storm that could knock out much of the nation's power grid. Or an earthquake could hit an East Coast city not generally considered vulnerable to a major temblor. That sounds like paranoia, but mainstream scientists and government officials research such things.

"South Carolina's got a very significant seismic history," Fugate noted. "There's a fault that runs through Charleston, South Carolina, that has devastated that area before." That 1886 event, with an estimated magnitude of 7.3, killed 60 people and was felt as far away as Wisconsin, Boston and Cuba.

That was what geologists call an intraplate earthquake, an event within one of the planet's major tectonic plates rather than along the margin, where earthquakes are easier to understand and anticipate. What causes these intraplate earthquakes is a thorny scientific question. Some intraplate faults might have a major quake every few thousand years, so infrequently that they are not in sync with the human time scale and do not factor significantly in the U.S. Geological Survey's seismic hazard maps.

Conversely, some hazards are well publicized at this point but highly unlikely for centuries to come, such as a full-blown eruption of the Yellowstone caldera, sometimes referred to as a supervolcano. If Yellowstone were to explode, it would be an event thousands of times more violent than the Mount St. Helens eruption, and its effect would be felt across much of the western United States. But it has been 640,000 years since the last such event and, although the caldera is active and generates swarms of small earthquakes, there is no sign that a major event will happen in the lifetime of anyone around today.

Some hazards become more prominent with new scientific research. Only in the past couple of decades have scientists come to appreciate the threat posed by the Cascadia subduction zone, a tectonic plate boundary running just off the coast of the Pacific Northwest, from Northern California to Vancouver Island. It could generate a mega-quake like the one that just hit Japan.